I don't think the current Tenebrium skill is comparable to a magic discipline or Blacksmithing. The latter still offer some benefits when boosted to 5 as opposed to Tenebrium. Whether you can make use of the highest level spells or superior weapons / armor made with Blacksmithing 5 is another matter. As of Tenebrium, however, it simply doesn't make sense to raise the skill beyond 3 at the moment.
I like the idea of it giving a small (Tenebrium) damage boost. Maybe 5% per rank?
